A guide to IRCS echelle spectroscopy exposure
times

Near-infrared wavelengths: JHK

At high spectroscopic resolution (R > 5000),
it is possible to work
between
the OH lines
which dominate the broad-band background flux from the night sky. The
brightness of the night-sky continuum is not well-determined, but measurements
suggest a value of ~ 1000 photons/s/m2/arcsec2/um
may be typical. The small pixels and high resolving power of the IRCS
echelle result in a background flux between the
OH lines which is less than the detector dark current. Observations at JHK (i.e., where the
thermal background is unimportant) are therefore always detector-limited and sky
subtraction is not required.

If OH lines should be subtracted clearly for your science, the longest exposuretimes we recommand are 900 s for J-band, 200 s for H-band 300 s for K-band.

Note that while increasing the exposure time will not
degrade the scientific
quality of your data, provided you are well away from OH lines
(which
will saturate), there is always a risk of instrument and/or software
failure
resulting in the loss of the most recent exposure. We therefore caution
against exposures longer than 900 seconds.

Thermal-infrared wavelengths: LM

Due to the high thermal background, observations become
background-limited
in about 30 seconds at L', and about 4 seconds at M'.
A point
source with L=1 saturates in about 1
second.

Non-linearity of the spectrograph detector

We recommand the signal level less than 6000 ADU (=22,800 e-) for a single exposure to achieve less than 1% non-linearity.

Echelle Exposure Time Calculator

Please use Echelle-ETC to estimate total exposure time for the IRCS Echelle mode.